Diodes. How big and how many do I need?

01-26-2010, 11:42 PM

I bought about a hundred cells. They are 6x6 inches. although I need a better multimeter in order to test them (I will have it soon) they are supposed to generate about 4.8 watts a piece at about .6 volts and 8 amps. I've been practicing soldering them together and I'm ready to get serious. I'm planning to building a panel of sixty cells, 6x10. I'm pretty sure I will wire them in series which should give me about 288 watts at 36 volts and 8 amps give or take a little here and there for resistance. My trouble is, with all my research I still can't figure out what I will need for diodes. I know that most good charge controllers have them to prevent my batteries from discharging back into the panel. But I also think I ought to have some inside the panel. Ive researched blocking diodes, bypassing diodes, and isolation diodes, among others. However, I can't seem to find any information describing what strength of diode I should use. I want to protect the cells from backflow and bypass cells that may be shadowed briefly. Does this make sense? Am I making a mountain out of a molehill? Also, how many should I use.

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Delta fox is right, your diodes need to be able to pass the 8 amps, not be borderline burn out at 8amps. Like buying tires for your car, with a 60 mph speed rating. They might not blow immediately when passing a truck at 65mph, but why risk it, get the 90mph tires, or the 14A diodes.

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Thanks for the advice. I wonder about how many to use. I want to prevent damage to the cells and prevent shadows from stopping the panel from functioning but, I don't want to nit pick too much. The more diodes I use the less watts I'll get out of the panel. Also, it would be alot of work if I had to use 60 diodes. Do you think I may be better off wiring it in a series parallel combo?

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you only need to use one . I put mine on the last of my positive side before it get conected to the juntion box. You need to worry about buying good Charge Controler and a good Inverter. Don't forget the fuses , Disconnect ,ect.

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Your panel will need bypass diodes. You will need one bypass diode for every 10 to 15 series connected cells. Maybe every 10 would be easier for you since you are doing 10x6. The bypass diodes will have a negligible effect on power output. A 10Amp diode would be fine. If you want to nitpick, you could pick out a diode with a minimal leakage current.

Commercial modules don't use many bypass diodes because of cost (typically they use only two).

If you are using a charge controller, you will not need a blocking diode.

If you are connecting this panel in parallel with other panels to a single charge controller, you should install a blocking diode on each panel.

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Um, while reading up on diodes, some sites say that a bypass diode is the same as blocking diode except the way it is installed. They show a picture of a blocking diode with a lead connected between two cells going to the diode, and then between the next set of cells, thus bypassing a cell or group of cells if need be. Is that correct, or do I need an entirely different type of cell. I would draw a picture but I don't know how. LOL

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A bypass diode is connected in parallel with the cells that it is protecting. Specifically, a bypass diode prevents a negative voltage across a cell.

A blocking diode is connected in series with the cells that it is protecting. A blocking diode prevents a negative current out of the panel (or in other words, power dissipation in the panel).

You could use the same physical diodes for both purposes. For a blocking diode, you should pick a diode with minimal forward voltage drop and reverse current. Typically, Schottky diodes are recommended.

For a bypass diode the most characteristic is minimal reverse current.

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So you're saying I could use Schottky diodes as bypass diodes within my panel. then I could use the same as a blocking diode between panels. Or would I need stronger diodes between panels when connecting them in parallel?